This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:
3GPP third generation partnership project
ACCS autonomous component carrier selection
BIM background interference matrix
CA carrier aggregation
CC component carrier
CSG closed subscriber group
DL downlink (eNB to UE)
eNB E-UTRAN Node B (base station of an LTE system)
E-UTRAN evolved UTRAN (also known as LTE)
HeNB LTE femto node
HNB WCDMA femto node
IMT international mobile telecommunications
ITU-R international telecommunication union-radio
LTE long term evolution
LTE-A LTE advanced
SINR signal to interference plus noise ratio
UE user equipment
UL uplink (UE to eNB)
UTRAN universal terrestrial radio access network
WCDMA wideband code division multiple access
In the communication system known as evolved UTRAN (E-UTRAN, also referred to as UTRAN-LTE, E-UTRA), the LTE Release 8 is completed, the LTE Release 9 is being standardized, and the LTE Release 10 is currently under development within the 3GPP. In LTE the downlink access technique is orthogonal frequency division multiple access OFDMA, and the uplink access technique is single carrier frequency division multiple access SC-FDMA. These access techniques are expected to continue in LTE Release 10.
Of particular interest herein are the further releases of 3GPP LTE targeted towards future IMT-Advanced systems, referred to herein for convenience simply as LTE-Advanced (LTE-A). LTE-A is directed toward extending and optimizing the 3GPP LTE Release 8 radio access technologies to provide higher data rates at very low cost. LTE-A will most likely be part of LTE Release 10. LTE-A is expected to use a mix of local area and wide area optimization techniques to fulfill the ITU-R requirements for IMT-Advanced while keeping the backward compatibility with LTE Release 8.
There is a bandwidth extension beyond 20 MHz in LTE-Advanced which is to be done via carrier aggregation (CA). This is shown conceptually at FIG. 1 in which there are five CCs or carrier frequency bandwidths of 20 MHz each that are aggregated to form one larger LTE-Advanced bandwidth of 100 MHz. Each carrier frequency bandwidth has DL and UL resources for enabling increased data rates such as for example by simultaneously scheduling an active UE across multiple carrier frequency bandwidths to better distribute traffic. Other embodiments of CA may have non-contiguous carrier frequency bandwidths and/or an asymmetric DL/UL CA which for example may be built by combining a frequency division duplex FDD carrier with a time division duplex TDD carrier. LTE-A is not the only CA-type system.
Another aspect being developed in LTE-A is the concept of heterogeneous networking, or HetNet for short. Adjacent cells cooperate to achieve more efficient use of scarce radio resources even if they are different wireless systems. For example, there may be femto-cells, sometimes termed home base stations (HeNB in LTE; HNB in WCDMA) or other networks of one cell or very limited geographic area, existing side by side with other femto-cells and with traditional network-operated cellular base stations/eNBs. These cells may cooperate to mitigate interference with one another, or at least positively limit their own interference to adjacent cells to avoid the greedy cell scenario in which one cell occupies more bandwidth resources than its traffic justifies, at the expense of an adjacent cell.
ACCS is one of the interference management schemes that is proposed for LTE-A. In ACCS the network access node makes its own selection as to which CC it will take into use at a given time, with consideration to not interfering too much with adjacent cells. Reference is made to co-owned US provisional patent application no. 61/309,044, filed on Mar. 1, 2010 and entitled “Enhanced Estimation of Uplink Interference Coupling” , which details a technique by which femto nodes determine interference coupling with adjacent cells for use in selecting which CC to take into use.
Typically the femto node will be given a set of candidate carrier frequency bandwidths (also termed component carriers CCs) from which to choose. Denote these frequency bandwidths as {f1, f2, . . . , fn}, where N is the number of carriers in the whole CA system (typically for WCDMA each carrier frequency bandwidth is 5 MHz and in LTE the carrier frequency bandwidths currently range from 1.4 MHz to 20 MHz). The given set may be all N CCs in the CA or it may be a subset of them. Upon powering on a femto node, it will have to autonomously select which carrier frequency bandwidth of its given set to use. To maximize the femto cell performance it has been recommended that the femto node measure the total received interference on each carrier, and then select the carrier frequency bandwidth with the lowest interference level.
This simple approach of selecting the carrier with the lowest interference might appear optimal from the individual node's perspective but can lead to problems when multiple femto nodes employ that same technique. Specifically, problems are likely to occur if many closed subscriber group CSG femto cells in a dense local area are using all of the possible carrier frequency bandwidths which are also available for macro cell users. In these instances a macro cell user, that is not part of any femto cell's CSG, will experience significant interference from the femto nodes. It is quite possible that the interference can be severe enough and the resulting SINR so poor that the macro cell cannot find for its own use a carrier frequency bandwidth that is sufficiently free of co-channel interference with femto cells, leading to what is termed a “macro cell coverage hole”.
Prior to this invention, the solution to this problem of which the inventors are aware was to restrict the set of candidate carrier frequency bandwidths given to the femto cells to ensure that one carrier for the macro operator is always free of CSG H(e)NB. One may make the offered frequency range different depending on whether the H(e)NB is of type closed (CSG), open (non-CSG) or hybrid. But for femto operators that only have (for example) two or three carriers available this approach to ensure full coverage on the macro layer is a severe limitation to radio performance on the femto layer. The above approach of reserving an “escape carrier” for macro use is quite expensive respecting scarce over the air radio resources, and unnecessary in many areas of the network. These teachings provide a more elegant solution that is not so restrictive to the femto layer.